Fix empty VITE_API_URL falling back to localhost in production

Use ?? instead of || so empty string (same-origin) is preserved
while undefined still falls back to localhost:3001 for dev.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/lib/api.ts

@@ -1,4 +1,4 @@
-const API_BASE = import.meta.env.VITE_API_URL || 'http://localhost:3001';
+const API_BASE = import.meta.env.VITE_API_URL ?? 'http://localhost:3001';
 
 let authToken: string | null = localStorage.getItem('ai-tycoon-auth-token');